Watch the Power Chords! online guitar lesson by Angus Clark from Essentials: Rock Rhythm Unplugged
The part of this lesson regarding the riff is fairly straightforward - play it clean!
The lesson regarding the chord voicings is a bit more in depth, and you can really get a lot of mileage out of this concept. Similar to the lessons regarding using the B and E strings as common tones across changing chords, here you're using the open G string as a common tone. Ty Tabor of King's X was a big fan of the open G string. And of course, if you use a capo, you can get even more mileage out of this concept on other material.
By using your thumb to play the bass notes on the sixth string, you're able to get your power chords and then add notes on the top three strings so that your rhythm part now has a much wider range than if you were just stuck on the bottom two or three strings. The right hand follows suit and now you can strum across all six strings generating more volume and energy.